1. Field of the Invention
The present invention relates to relay stations for use in a telecommunications transmission system.
2. Description of the Prior Art
In transmission systems which operate using a radio link, it is frequently impossible to establish a direct radio link between the two terminal stations. In such cases, the radio link is split into two or more sections which are in each case connected by a relay station. These relay stations receive from the preceding (in the direction of transmission) radio link section the radio frequency signals modulated by the information, and relay them to the succeeding (in the direction of transmission) radio link section, and thus to the next relay station or terminal station, in the form of radio frequency signals. For reasons associated with decoupling the receiving section from the transmitting section in a relay station, in the latter, the radio frequency signals of the transmitting section are shifted in frequency with respect to those of the receiving section. In radio relay links, particularly those operating over very long distances, instead of the large number of relay stations which would otherwise be required and would be connected across the surface of the earth in a chain, frequently a satellite link is used in which the transmitting ground terminal station sends its radio frequency signals to a relay station contained in the satellite at an appropriate height above the earth's surface. The satellite relay station radiates these radio frequency signals at a different frequency to another ground station or terminal station. In radio relay links of this kind, the receivers are designed as superheterodyne receivers and the transmitters as superheterodyne transmitters. The connection of the receivers with the transmitters of the relay station is generally effected by providing that the received radio frequency signal is transposed to a lower, intermediate frequency and thence converted by a superheterodyne transmitter to the radio frequency which is to be used for transmission. For this purpose, the frequency band of the intermediate frequency signal is split into several adjacent sub-bands by means of frequency multiplexer filters, in systems which have several transmission channels. The sub-bands are in each case independently amplified and, using a suitable carrier frequency, are converted in a corresponding number of superheterodyne transmitters to the radio frequency which is to be used for transmission. Because the carrier frequency is the same for all the superheterodyne transmitters, the individual radio frequency sub-bands obtained in this manner are united to form the overall transmitted radio frequency band.
In satellite communications relay stations, the further factor has to be borne in mind that for reasons of reliability the corresponding units must be designed so that their active sections incorporate a redundancy and accordingly, in the case of the superheterodyne transmitter in particular, a considerable amount of carrier power is required. In the context of transpondence of this kind, it has been provided that the carrier frequencies required for the individual superheterodyne stages be produced by separate oscillators, these oscillators controlling the desired frequencies by means of special synchronizing devices, in particular an injection synchronizing system.
In the context of earth-bound radio relay systems for use by public communication systems, a system which differs from this is employed. In these systems, the individual relay station operates with a single intermediate frequency band. If more channel capacity is required, several radio relay links, i.e., several radio frequency channels, are combined to form a large group in which the channels are disposed adjacent to one another in a relatively narrow radio frequency band. Accordingly, in these systems, a large number of carrier frequencies are required for frequency transposition by the different superheterodyne receivers and superheterodyne transmitters. These carrier frequencies must have high frequency stability. The circuits for generating these carrier oscillations are accordingly relatively elaborate where systems of any size are concerned. In some radio relay systems, therefore, the technique of a so-called central carrier supply has been adopted in which, commencing from several master generators, the corresponding carrier frequencies are produced by frequency conversion operations using preceding and succeeding filters. An example of this kind of carrier supply arrangement is described in the magazine "The Bell System Technical Journal," 1961, Volume 40, pages 1569 to 1586.